Treacle’s ability to form liquid phase condensates is essential for nucleolar fibrillar center assembly, efficient rRNA transcription and processing, and rRNA gene repair

Abstract

We investigated the role of the nucleolar protein Treacle in organizing and regulating the nucleolus in human cells. Our results support Treacle’s capacity to form liquid-phase condensates through electrostatic interactions among molecules. The formation of these biomolecular condensates is crucial for segregating nucleolar fibrillar centers from the dense fibrillar component, as well as ensuring high levels of rRNA gene transcription and accurate pre-rRNA processing. The presence of both the central and C-terminal domains of Treacle is necessary for the creation of liquid-phase condensates. Initiation of phase separation is attributed to the C-terminal domain, while the central domain, characterized by repeated stretches of alternatively charged amino-acid residues, is vital for maintaining the condensate’s liquid state. When mutant forms of Treacle, incapable of forming liquid-phase condensates, are overexpressed in cells, it compromises the establishment of fibrillar centers, leading to the suppression of rRNA transcription and disruption of its processing. Additionally, these mutant forms fail to recruit TOPBP1, resulting in the suppression of the DNA damage response in the nucleolus.